The present invention relates to light emitting diodes (LEDs) and, more particularly, to a phosphor-converted LED device that utilizes one or more phosphor thin films for converting primary light emitted by the LED into one or more other frequencies of light in order to generate white light.
With the development of efficient LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emission of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelength to produce white light. LEDs that produce white light are useful for signaling and/or illumination purposes.
Currently, state-of-the-art phosphor conversion of a portion of the primary emission of the LED is attained by placing a phosphor in an epoxy that is used to fill the reflector cup, which houses the LED within the LED lamp. The phosphor is in the form of a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED and is subsequently cured.
The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary light emitted by the LED passes through the epoxy without impinging on the phosphor particles, whereas a portion of the primary light emitted by the LED impinges on the phosphor particles, thereby causing the phosphor particles to emit complimentary light. The combination of the primary blue light and the phosphor-emitted light produces white light. One disadvantage of utilizing the epoxy comprising the phosphor particles is that uniformity in the white light emitted by the LED is difficult, if not impossible, to obtain. This non-uniformity is caused by non-uniformity in the sizes of the phosphor particles mixed into the epoxy slurry. Currently, phosphor powders having uniform phosphor particle sizes generally are not available. When the phosphor powder is mixed into the epoxy slurry, the larger phosphor particles sink faster than the smaller phosphor particles. This non-uniformity in spatial distribution of the phosphor particles exists in the epoxy once it has been cured.
Therefore, obtaining a uniform distribution of the phosphor particles within the epoxy is very difficult, if not impossible, due to the non-uniformity of the sizes of the phosphor particles. This inability to control the sizes of the phosphor particles and their locations within the epoxy results in difficulties in producing LED lamps that emit white light in a consistent manner. Therefore, the quality of the white light produced by LED lamps may vary from one lamp to another, even for a given model manufactured by a particular manufacturer.
Attempts have been made to overcome the disadvantages of using phosphor powders mixed with epoxies by placing luminescent organic dye films on a lens that encases the LED. The dye is carefully positioned on the lens at a particular location so that it totally absorbs all of the primary light impinging thereon and converts the primary light to light of a longer wavelength. A fraction of the primary light emitted passes through the lens without impinging on the dye. The primary light that does not impinge on the dye then combines with the longer-wavelength light to produce white light. Since the dye totally absorbs the primary light impinging thereon, any variation in the fraction of the primary light that is summed with the longer-wavelength light is supposed to be eliminated.
However, this latter approach also has several disadvantages. The placement of the dye on the lens is subject to manufacturing uncertainties, which may result in variations in the white light produced. Also, dyes that are stable over long periods of time generally are not available. As a result, wide spread use of wavelength-converting dyes has not occurred.
Accordingly, a need exists for a phosphor-converted LED that overcomes these problems and disadvantages.
The present invention provides a phosphor-converted LED device comprising one or more phosphor thin films that convert primary light emitted by the LED into one or more other wavelengths of light to produce light of a particular color. The phosphor thin film of the invention comprises dopants that are spatially distributed in an optically homogeneous manner. This optically-homogeneous spatial distribution of the dopants in the thin film causes the phosphor conversion of the primary light to occur in a manner that is predictable and controllable. Therefore, variations in the color of the light produced by phosphor-converted LED devices can be eliminated, thereby enabling consistency in the quality of the colored light produced by LED devices to be achieved. Preferably, the light of the particular color that is produced by the LED device is white light. However, the phosphor-converted LED device of the present invention may be designed to perform phosphor conversion to produce light of other colors, such as green light.
In accordance with one embodiment of the present invention, a single phosphor thin film is comprised by the LED device. The thin film converts a portion of the blue primary light into yellow light while allowing a portion of the primary light to pass through the thin film. The primary light and the yellow light combine to produce white light.
In accordance with an alternative embodiment of the present invention, at least two phosphor thin films, each of which emits a different color of light, are comprised in the LED device. In accordance with this embodiment, the thin films preferably are disposed one on top of the other. Each of the thin films converts a portion of the primary light into a particular color of light. For example, one of the thin films may convert primary light into green light and one of the films may convert primary light into red light. The red light, green light and the unconverted primary light combine to form white light.
In accordance with another embodiment of the present invention, at least two phosphor thin films are utilized and one of the thin films completely absorbs the primary light and converts it into blue light. The other thin film then converts a portion of the blue light emitted by the first thin film into a broad yellow emission, which then combines with the blue emission to produce white light.
Other features and advantages of the present invention and variations thereof will become apparent from the following description, drawings and claims.